High fatigue strength steel sheet excellent in burring workability and method for producing the same

ABSTRACT

A compound structure steel sheet excellent in burring workability made of a steel containing, by mass, 0.01 to 0.3% of C, 0.01 to 2% of Si, 0.05 to 3% of Mn, 0.1% or less of P, 0.01% or less of S, and 0.005 to 1% or Al, and having the microstructure being a compound structure having ferrite as the main phase and martensite or retained austenite mainly as the second phase, the quotient of the volume percentage of the second phase divided by the average grain size of the second phase being 3 or more and 12 or less, and the quotient of the average hardness of the second phase divided by the average hardness of the ferrite being 1.5 or more and 7 or less; or a compound structure steel sheet excellent in burring workability made of a steel containing, by mass, 0.01 to 0.3% of C, 0.01 to 2% of Si, 0.05 to 3% of Mn, 0.1% or less of P, 0.01% or less of S, and 0.005 to 1% or Al, having the microstructure being a compound structure having ferrite as the main phase and martensite or retained austenite mainly as the second phase, the average grain size of the ferrite being 2 μm or more and 20 μm or less, the quotient of the average grain size of the second phase divided by the average grain size of the ferrite being 0.05 or more and 0.8 or less, and the carbon concentration in the second phase being 0.2% or more and 3% or less.

TECHNICAL FIELD

[0001] This invention relates to a compound structure steel sheetexcellent in burring workability, having a tensile strength of 540 MPaor more, and a method to produce the same, and, more specifically, to ahigh fatigue strength steel sheet excellent in hole expansibility(burring workability) and suitable as a material for roadwheels andother undercarriage parts of cars wherein both the hole expansibilityand durability are required, and a method to produce the same.

BACKGROUND ART

[0002] The application of light metals such as aluminum alloys and highstrength steel sheets to car components is being increased to achievefuel economy and other related advantages through car weight reduction.Although light metals such as aluminum alloys have an advantage of highspecific strength, their application is limited to special uses becauseof a far higher cost than steel. To further reduce car weight,therefore, a wider application of low cost, high strength steel sheetsis required.

[0003] Facing the demands for higher strength, against the abovebackground, various new steel sheets having high strength, deepdrawability, bake-hardenability, etc. have so far been developed in thefield of cold-rolled steel sheets used for bodies and panels, whichaccount for a quarter or so of the total car weight, and thesedevelopments have contributed to the reduction in car weight. The focusof efforts for car weight reduction, however, has lately shifted tostructural members and undercarriage components, which account for about20% of the total car weight. In this situation, immediate action isdemanded in the development of high strength hot-rolled steel sheets forthese applications.

[0004] However, generally speaking, high strength is obtained at a costof other material properties such as formability (workability) and,therefore, the key issue in the development of the high strength steelsheets is how to raise steel strength without sacrificing other materialproperties. Hole expansibility, fatigue resistance, corrosion resistanceand the like are important among the properties required of steel sheetsused especially for structural members and undercarriage components. Itis essential, in this development, to realize high strength togetherwith high values of these properties in a well-balanced manner.

[0005] Among the properties required of the steel sheets for roadwheeldiscs, for example, hole expansibility and fatigue resistance areregarded as particularly important. This is because burring (holeexpansion) to form a hub hole is especially difficult, among variousworking stages, in forming a roadwheel disc and the fatigue resistanceis the aspect controlled under the most stringent standards among theproperties required of wheel components.

[0006] In consideration of the fatigue resistance of the wheelcomponents, high strength hot-rolled steel sheets of 590 MPa classferrite-martensite compound structure steel (the so-called dual-phasesteel) excellent in fatigue property are presently used for theroadwheel discs. The level of strength required of the steel sheets forthese components, however, is rising yet further from the 590 MPa classto the 780 MPa class. In addition to the fact that the holeexpansibility tends to lower as the steel strength increases, thecompound structure steel sheets are believed to be handicapped withregard to the hole expansibility because of their inhomogeneousstructure. For this reason, the hole expansibility, which does notconstitute any problem in the 590 MPa class compound structure steelsheets, may become a problem with 780 MPa class compound structure steelsheets.

[0007] This means that the hole expansibility is highlighted, inaddition to the fatigue resistance, as an important subject in theapplication of high strength steel sheets to roadwheels and otherundercarriage components of cars. However, despite the strong demands,few inventions have been proposed, save for a limited number ofexceptions, to provide high strength steel sheets having amicrostructure of a ferrite-martensite compound structure to improve thefatigue resistance, and which are also excellent in hole expansibility.

[0008] Japanese Unexamined Patent Publication No. H5-179396, forexample, discloses a technology to secure the fatigue resistance of asteel sheet by forming its microstructure to consist of ferrite andmartensite or retained austenite, and to ensure the hole expansibilityby strengthening ferrite with precipitates of TiC, NbC, etc. so that thestrength difference between ferrite grains and a martensite phase may bedecreased and deformation may not concentrate locally on ferrite grains.

[0009] In the steel sheets for some of the undercarriage components suchas roadwheel discs, it is essential to realize a well-balanced andhigh-level combination of formability such as burring workability andfatigue resistance, but the above technology does not offer theseproperties in a satisfactory manner. Besides, even if both theformability and fatigue resistance are satisfactory, it is important toprovide a production method capable of providing these featureseconomically and stably and, in this respect, the above conventionaltechnology is insufficient.

[0010] To be more specific, the technology disclosed in JapaneseUnexamined Patent Publication No. H5-179396 is incapable of providing asufficient elongation because it proposes to strengthen the ferritegrains by precipitation hardening. Nor is it capable of providing a lowyield ratio, which is a unique characteristic of the ferrite-martensitecompound structure, because the precipitates block movable, high-densitydislocations created around the martensite phase during production.Besides, the addition of Ti and Nb is not desirable since it raisesproduction costs.

[0011] In view of the above, the object of the present invention is toprovide a compound structure steel sheet capable of advantageouslysolving the above problems of conventional technologies, excellent infatigue resistance and burring workability (hole expansibility) andhaving a tensile strength of 540 MPa or more, and a method to producesaid steel sheet economically and stably.

DISCLOSURE OF THE INVENTION

[0012] Keeping in mind the production processes of hot-rolled andcold-rolled steel sheets presently produced on an industrial scale usinggenerally employed steel sheet production facilities, the presentinventors earnestly studied the means to achieve both good burringworkability and high fatigue resistance of steel sheets. As a result,the present invention was established based on the new discovery thatachieving the following was very effective for enhancing the burringworkability: that microstructure is a compound structure having ferriteas the main phase and martensite or retained austenite mainly as thesecond phase; that the average grain size of the ferrite is 2 μm or moreand 20 μm or less, that the quotient of the average grain size of thesecond phase divided by the average grain size of the ferrite is 0.05 ormore and 0.8 or less, and that the carbon concentration of the secondphase is 0.2% or more and 2% or less; that the quotient of the volumepercentage of the second phase divided by the average grain size of thesecond phase is 3 or more and 12 or less; and that the quotient of theaverage hardness of the second phase divided by the average hardness ofthe ferrite is 1.5 or more and 7 or less.

[0013] The gist of the present invention, therefore, is as follows:

[0014] (1) A high fatigue strength steel sheet excellent in burringworkability characterized in that: the steel sheet is made of a steelcontaining, in mass,

[0015] 0.01 to 0.3% of C,

[0016] 0.01 to 2% of Si,

[0017] 0.05 to 3% of Mn,

[0018] 0.1% or less of P,

[0019] 0.01% or less of S, and

[0020] 0.005 to 1% or Al, and

[0021] the balance consisting of Fe and unavoidable impurities; themicrostructure is a compound structure having ferrite as the main phaseand martensite as the second phase; the average grain size of theferrite is 2 μm or more and 20 μm or less;

[0022] the quotient of the average grain size of the second phasedivided by the average grain size of the ferrite is 0.05 or more and 0.8or less; and the carbon concentration in the second phase is 0.2% ormore and 3% or less.

[0023] (2) A high fatigue strength steel sheet excellent in burringworkability characterized in that: the steel sheet is made of a steelcontaining, in mass,

[0024] 0.01 to 0.3% of C,

[0025] 0.01 to 2% of Si,

[0026] 0.05 to 3% of Mn,

[0027] 0.1% or less of P,

[0028] 0.01% or less of S, and

[0029] 0.005 to 1% or Al, and

[0030] the balance consisting of Fe and unavoidable impurities;

[0031] the microstructure is a compound structure having ferrite as themain phase and martensite as the second phase; the quotient of thevolume percentage of the second phase divided by its average grain sizeis 3 or more and 12 or less; and

[0032] the quotient of the average hardness of the second phase dividedby the average hardness of the ferrite is 1.5 or more and 7 or less.

[0033] (3) A high fatigue strength steel sheet excellent in burringworkability characterized in that; the steel according to the item (1)or (2) further contains, in mass, 0.2 to 2% of Cu, and the Cu exists inthe ferrite phase of the steel in the state of the precipitates ofgrains 2 nm or less in size consisting purely of Cu and/or in the stateof solid solution.

[0034] (4) A high fatigue strength steel sheet excellent in burringworkability characterized in that the steel according to any one of theitems (1) to (3) further contains, in mass, 0.0002 to 0.002% of B.

[0035] (5) A high fatigue strength steel sheet excellent in burringworkability characterized in that the steel according to any one of theitems (1) to (4) further contains, in mass, 0.1 to 1% of Ni.

[0036] (6) A high fatigue strength steel sheet excellent in burringworkability characterized in that the steel according to any one of theitems (1) to (5) further contains, in mass, one or both of 0.0005 to0.002% of Ca and 0.0005 to 0.02% of REM.

[0037] (7) A high fatigue strength steel sheet excellent in burringworkability characterized in that the steel according to any one of theitems (1) to (6) further contains, in mass, one or more of;

[0038] 0.05 to 0.5% of Ti,

[0039] 0.01 to 0.5% of Nb,

[0040] 0.05 to 1% of Mo,

[0041] 0.02 to 0.2% of V,

[0042] 0.01 to 1% of Cr, and

[0043] 0.02 to 0.2% of Zr.

[0044] (8) A high fatigue strength steel sheet excellent in burringworkability characterized in that; the steel sheet is made of a steelhaving the chemical composition according to any one of the items (1) to(7), and the microstructure is a compound structure having ferrite asthe main phase and retained austenite accounting for a volume percentageof 5% or more and 25% or less as the second phase.

[0045] (9) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by, when hot rolling aslab having the chemical composition according to any one of the items(1) to (7), completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of 350° C. or lower.

[0046] (10) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by, when hot rolling aslab having the chemical composition according to any one of the items(1) to (7), applying high pressure descaling to the slab after roughrolling, completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of 350° C. or lower.

[0047] (11) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by completing the hotrolling of a slab having the chemical composition according to any oneof the items (1) to (7) at a temperature of the Ar₃ transformationtemperature or higher, subsequently pickling and cold-rolling thehot-rolled steel sheet thus produced, holding the cold-rolled steelsheet in the temperature range from the Ac₁ transformation temperatureto the Ac₃ transformation temperature for 30 to 150 sec., and thencooling it at a cooling rate of 20° C./sec. or higher to the temperaturerange of 350° C. or lower.

[0048] (12) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by, when hot rolling aslab having the chemical composition according to any one of the items(1) to (7), completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of above 350° C. and 450° C. or lower.

[0049] (13) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by, when hot rolling aslab having the chemical composition according to any one of the items(1) to (7), applying high pressure descaling to the slab after roughrolling, completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of above 350° C. and 450° C. or lower.

[0050] (14) A method to produce a high fatigue strength steel sheetexcellent in burring workability characterized by, completing the hotrolling of a slab having the chemical composition according to any oneof the items (1) to (7) at a temperature of the Ar₃ transformationtemperature or higher, subsequently pickling and cold rolling thehot-rolled steel sheet thus produced, holding the cold-rolled steelsheet in the temperature range from the Ac₁ transformation temperatureto the Ac₃ transformation temperature for 30 to 150 sec., then coolingit at a cooling rate of 20° C./sec. or higher, holding it in thetemperature range of above 350° C. and 450° C. or lower for 15 to 600sec., and cooling it at a cooling rate of 5° C./sec. or higher to thetemperature range of 150° C. or below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051]FIG. 1 is a graph showing the relationship between an averageferrite grain size, the size of second phase and a hole expansion rateobtained from the result of a preliminary test for the presentinvention.

[0052]FIG. 2 is a graph showing the relationship between carbonconcentration in the second phase and a hole expansion rate obtainedfrom the result of a preliminary test for the present invention.

[0053]FIG. 3 is a graph showing the relationship between the quotient ofthe volume percentage of the second phase divided by the average grainsize of the second phase, the quotient of the average hardness of thesecond phase divided by the average hardness of the ferrite and a holeexpansion rate obtained from the result of a preliminary test for thepresent invention.

[0054]FIG. 4 is a view showing the shape of a test piece for a fatiguetest.

BEST MODE FOR CARRYING OUT THE INVENTION

[0055] The results of the fundamental researches which led to thepresent invention will be described.

[0056] The influence of the average grain size of the ferrite and thesize of the second phase on hole expansibility was investigated first.The specimens for the test were prepared in the following manner:completing the finish hot rolling of steel slabs having the chemicalcompositions of 0.07% C-1.6% Si-2.0% Mn-0.01% P-0.001% S-0.03% Al atdifferent temperatures of the Ar₃ transformation temperature or above,holding the hot-rolled sheets thus produced in different temperatureranges from the Ar₁ transformation temperature to the Ar₃ transformationtemperature for 1 to 15 sec., cooling at a cooling rate of 20° C./sec.or higher, and then coiling at an ordinary temperature.

[0057]FIG. 1 shows the result of the hole expanding test of the steelsheets thus prepared in relation to the average grain size of theferrite and the size of the second phase.

[0058] From the result, the present inventors newly discovered thatthere was a strong correlation between hole expansibility and each ofthe average grain size of the ferrite and the size of the second phase(the quotient of the average grain size of the second phase divided bythe average grain size of the ferrite), and that the hole expansibilitywas markedly enhanced when the average grain size of the ferrite was 2μm or more and 20 μm or less and the quotient of the average grain sizeof the second phase divided by the average grain size of the ferrite is0.05 or more and 0.8 or less.

[0059] The mechanism for this is not altogether clear, but it issupposed to be as follows: if the size of the second phase is too large,voids form easily at the interface between the second phase and itsparent phase and the voids serve as initial points of cracks during holeexpansion; if it is too small, local ductility, which correlates withthe hole expansion rate, is lowered; and thus the hole expansion rateincreases when the second phase has the optimum size and interval. It isalso supposed that, if the average grain size of the ferrite is toosmall, yield stress increases adversely affecting the shape-freezingproperty after forming, and if it is too large, the microstructurebecomes inhomogeneous and local ductility, which correlates with thehole expansion rate, is lowered.

[0060] Note that the average grain size of ferrite was measured inaccordance with the section method stipulated in the test method offerrite crystal grain size of JIS G 0552 steel, and that the averagegrain size of the second phase was defined as the equivalent diameter ofan average circle and the value obtained from an image processor and thelike was used.

[0061] Then, the influence of the carbon concentration in the secondphase on the hole expansibility was investigated. FIG. 2 shows the holeexpansibility of the above steel sheets in relation to the carbonconcentration in the second phase. The present inventors newlydiscovered from the result that there was a strong correlation betweenthe carbon concentration in the second phase and the hole expansibilityand that, when the carbon concentration in the second phase was 0.2% ormore and 2% or less, the hole expansibility was markedly improved.

[0062] The mechanism for this is not altogether clear either, but it issupposed to be as follows: if the carbon concentration in the secondphase is too high, the strength difference between the second phase andits parent phase becomes large and, as a result, voids form easily atthe interface between them during punching work and the voids serve asinitial points of cracks during hole expansion; if the carbonconcentration in the second phase is too low, on the other hand, theductility of the ferrite phase inevitably lowers and local ductility,which correlates with the hole expansion rate, lowers and the holeexpansion rate decreases; and thus the hole expansion rate increaseswhen the carbon concentration in the second phase assumes an optimumvalue.

[0063] If the carbon concentration in the second phase exceeds 1.2%,however, heat affected zones soften remarkably during welding by spotwelding or similar methods and the softened heat affected zones maytrigger fatigue failures. For this reason, it is preferable that thecarbon concentration in the second phase falls within the range from 0.2to 1.2%.

[0064] Note that the hole expansibility (burring workability) wasevaluated following the hole expanding test method according to theJapan Iron and Steel Federation Standard JFS T 1001-1996.

[0065] Next, the microstructure and the carbon concentration in thesecond phase of a steel sheet according to the present invention will beexplained in detail.

[0066] To obtain good values in both the fatigue property and theburring workability (hole expansibility), the microstructure of a steelsheet according to the present invention is defined to be a compoundstructure having ferrite as the main phase and martensite or retainedaustenite mainly as the second phase. Note that the second phase maycontain unavoidable bainite and pearlite.

[0067] Here, the volume percentages of the retained austenite, ferrite,bainite, pearlite and martensite are defined as the respective areapercentages observed by a optical microscope at a magnification of 200to 500 times in the microstructure on the section surface at ¼ of thesheet thickness of the specimens cut out from the ¼ or ¾ width positionof the steel sheets, after polishing the section surface along therolling direction and etching it with a nitral reagent and a reagentdisclosed in Japanese Unexamined Patent Publication No. H5-163590.

[0068] Austenite can easily be identified crystallographically becauseits crystal structure is different from that of ferrite. The volumepercentage of the retained austenite can therefore be obtainedexperimentally by the X-ray diffraction method. This is a simplifiedmethod to calculate the volume percentages of austenite and ferrite fromthe difference between the two in the reflection surface intensity underirradiation by Kα-rays of Mo, using the following equation:

Vγ=(⅔){100/(0.7×α(211)/γ(220)+1)}+(⅓){100/(0.78×α(211)/γ(311)+1)},

[0069] where, α(211), γ(220) and γ(311) are the X-ray reflection surfaceintensities of ferrite (α) and austenite (γ), respectively.

[0070] Since the optical microscope observation and the X-raydiffraction method yield nearly identical measurements of the volumepercentage of the retained austenite, either of the measurements may beused.

[0071] The carbon concentration in the retained austenite can beobtained experimentally by either the X-ray diffraction method or byMössbauer spectrometry. By the X-ray diffraction method, for example,the carbon concentration in the retained austenite can be measured fromthe relationship between the carbon concentration and the change inlattice constant caused by the placement of C, an interstitial solidsolution element, at the crystal lattice of austenite. The latticeconstant is obtained by measuring the angles of reflection of (002),(022), (113) and (222) planes of austenite using Kα-rays of Co, Cu andFe, and calculating it from the angle of reflection described in aliterature (B. D. Cullity: Fundamentals of X-ray Diffraction, translatedby Gentaro Matsumura, published by Agne). Here, since there is a linearcorrelation between COS²θ(θ: angle of reflection) and lattice constanta, true lattice constant a₀ is obtained by extrapolating cos²θ=0 withthe straight line. The carbon concentration in the retained austenitecan be obtained also from the value of the true lattice constant a₀using the relationship between the lattice constant of austenite and thecarbon concentration in the austenite such as equation a₀=3.572+0.033% C(carbon concentration) described in the literature (R. C. Ruhl and M.Cohen: Transaction of the Metallurgical Society of AIME, vol. 245 (1969)p241).

[0072] If the second phase is martensite, then the carbon concentrationin the second phase is the value obtained by the calibration curvemethod described in a literature (Hiroyoshi Soejima: Electron Beam MicroAnalysis, published from Nikkan Kogyo Shimbunsha) using an electronprobe micro analyzer (EPMA). Note that, because five or more of thesecond phase grains were measured, the carbon concentration value is anaverage value of the measured grains. The carbon concentration in theretained austenite may be obtained by the following simplified measuringmethod as a substitution to the above methods, namely a method tocalculate it from the carbon content of the entire steel (the phasehaving the largest volume percentage and the second phase), which is theaverage carbon concentration in the entire steel, and the carbonconcentration in the ferrite.

[0073] The carbon content of all the steel (the phase having the largestvolume percentage and the second phase) is the carbon content in steelchemical composition, and the carbon concentration in the ferrite can becalculated from a bake-hardenability index (hereinafter BH). Note thatthe amount of BH (MPa) here is the value obtained by giving a 2.0%pre-strain to a JIS No. 5 test piece for tensile test, heat-treating itat 170° C. for 20 min. and conducting a tensile test again, which valuerepresents the difference between the flow stress under the 2.0%pre-strain before the heat treatment and the yield point after the heattreatment.

[0074] The BH amount of a compound structure steel may be regarded tocorrelate to the solute carbon amount in ferrite, since it is safe toconsider that the hard second phase does not deform plastically under apre-strain of 2.0% or so.

[0075] The relationship between the solute carbon amount and the BHamount of compound structure steels is shown in the literature (A. T.Davenport: Formable HSLA and Dual-Phase Steels (1977), FIG. 4 on p.131).From the relationship given therein, the relationship between the BHamount and the solute carbon amount of compound structure steels can beapproximated as follows:

Cs (solute carbon amount)=1.5×10⁻⁴ exp(0.033×BH).

[0076] The carbon concentration in the second phase can, therefore, beestimated by the following equation:

Cm=[C (carbon content of steel)−Cs]/fM (volume percentage of the secondphase).

[0077] There is a very good correlation between the carbon concentrationin the second phase estimated by the above equation and the sameobtained using EPMA.

[0078]FIG. 3 shows the result of the hole expanding tests of the steelsheets in terms of the quotient of the volume percentage of the secondphase Vs divided by the average grain size of the second phase dm andthe quotient of the average hardness of the second phase Hvs divided bythe average hardness of the ferrite Hvf.

[0079] From this, the present inventors discovered that there was astrong correlation between hole expansibility and each of the quotientof the volume percentage of the second phase divided by the averagegrain size of the second phase and the quotient of the average hardnessof the second phase divided by the average hardness of the ferrite, andthat the hole expansibility improved remarkably when the quotient of thevolume percentage of the second phase divided by the average grain sizeof the second phase was 3 or more and 12 or less and the quotient of theaverage hardness of the second phase divided by the average hardness offerrite was 1.5 or more and 7 or less.

[0080] The mechanism for this is not altogether clear either, but it issupposed to be as follows: if the quotient of the volume percentage ofthe second phase divided by the average grain size of the second phase(which quotient represents the grain size of the second phase) is toolarge, then the microstructure becomes inhomogeneous and voids arelikely to form at the interface between the second phase and its parentphase, and the voids are likely to initiate cracks during holeexpansion; if the above quotient is too small, local ductility, whichcorrelates with the hole expansion rate, is lowered; and thus the holeexpansion rate increases when the quotient assumes an optimum value.

[0081] It is also supposed that, if the quotient of the average hardnessof the second phase divided by the average hardness of the ferrite(which quotient represents the hardness difference between the ferriteand the second phase) is too large, voids are likely to form at theinterface between the second phase and its parent phase and the voidsare likely to initiate cracks during hole expanding, and that, if theabove quotient is too small, the effect of the second phase to arrestfatigue cracks is lost and, thus, it becomes difficult to obtain a goodhole expansibility and a good fatigue property at the same time.

[0082] The reasons for the definition of the chemical composition of asteel sheet according to the present invention will be explained. Thecontent of each of the elements is defined in mass.

[0083] C is indispensable for obtaining a desired microstructure. Whenits content exceeds 0.3%, however, it deteriorates workability andweldability and, hence, its content has to be 0.3% or less. When the Ccontent is below 0.01%, steel strength decreases and, therefore, itscontent has to be 0.01% or more.

[0084] Si is indispensable for obtaining a desired microstructure, andis effective for enhancing strength through solid solution hardening.Its content has to be 0.01% or more for obtaining a desired strengthbut, when contained in excess of 2%, it deteriorates workability. The Sicontent, therefore, has to be 0.01% or more and 2% or less.

[0085] Mn is effective for enhancing strength through solid solutionhardening. Its content has to be 0.05% or more for obtaining a desiredstrength but, when added in excess of 3%, cracks occur in slabs. Thusits content has to be 3% or less.

[0086] P is an undesirable impurity and the lower its content, thebetter. When its content exceeds 0.1%, workability and weldability areadversely affected, and so is fatigue property. Therefore, its contenthas to be 0.1% or less.

[0087] S is an undesirable impurity and the lower its content, thebetter. When its content is too large, the A type inclusions detrimentalto the hole expansibility are formed and, for this reason, its contenthas to be minimized. An S content of 0.01% or less is permissible.

[0088] 0.005% or more of Al is required for the deoxidation of moltensteel but its upper limit is set at 1% to avoid a cost increase. Alincreases the formation of non-metallic inclusions and deteriorateselongation when added excessively and, for this reason, a preferablecontent of Al is 0.5% or less.

[0089] Cu is added in an appropriate amount since, in solid solution, itimproves the fatigue property. However, a tangible effect is notobtained with an addition amount of below 0.2%, but the effect saturateswhen contained in excess of 2%. Thus, the range of the Cu content has tobe from 0.2 to 2%.

[0090] B is added in an appropriate amount since it raises fatigue limitwhen added in combination with Cu. An addition below 0.0002% is notenough to obtain the effect but, when added in excess of 0.002%, cracksare likely to occur in slabs. Hence, the B addition has to be 0.0002% ormore and 0.002% or less.

[0091] An appropriate amount of Ni is added for preventing hot shortnesscaused by Cu. An addition below 0.1% is not enough to obtain the effectbut, when added in excess of 1%, the effect saturates. For this reasonits content has to be 0.1 to 1%.

[0092] Ca and REM change the shape of non-metallic inclusions, whichinitiate fractures and deteriorate workability, and render themharmless. But a tangible effect is not obtained when each of theaddition amount is below 0.0005%. When Ca is added in excess of 0.002%or REM in excess of 0.02%, the effect saturates. Thus, it is preferableto add 0.0005 to 0.002% of Ca or 0.0005 to 0.02% of REM.

[0093] Additionally, precipitation hardening elements and/or solutionhardening elements, namely one or more of Ti, Nb, Mo, V, Cr and zr, maybe added to enhance strength. However, when the addition amount is below0.05%, 0.01%, 0.05%, 0.02%, 0.01% and 0.02%, respectively, no tangibleeffect shows and, when added in excess of 0.5%, 0.5%, 1%, 0.2%, 1% and0.2%, respectively, the effect saturates.

[0094] To obtain the effect of the present invention, no specific limithas to be set regarding Sn but, to avoid the occurrence of surfacedefects during hot rolling, it is preferable to limit its content to0.05% or less.

[0095] Now, the reasons for defining the conditions of the productionmethod according to the present invention will be described hereafter indetail.

[0096] In the present invention, slabs cast from molten steel preparedso as to contain the desired amounts of the component elements may befed directly to a hot rolling mill while they are hot or fed to a hotrolling mill after being cooled to room temperature and then heating ina reheating furnace. No specific limit is set regarding the reheatingtemperature, but it is desirable that the reheating temperature is below1,400° C. since, when it is 1,400° C. or higher, the amount of scale offbecomes large and the product yield is reduced. It is also desirablethat the reheating temperature is 1,000° C. or higher since a slabtemperature below 1,000° C. remarkably lowers the operation efficiencyof the mill in relation to its rolling schedule.

[0097] At finish rolling succeeding rough rolling in the hot rollingprocess, the rolling has to be completed at a final rolling temperature(FT) within the range from the Ar₃ transformation temperature to 100° C.above the Ar₃ transformation temperature. This is because, if therolling temperature falls below the Ar₃ transformation temperatureduring hot rolling, strain remains in the steel sheet, its ductility islowered, and thus workability is deteriorated, and, if the rollingcompletion temperature rises to more than 100° C. above the Ar₃transformation temperature, the austenite grain size after the finishrolling becomes too large, causing insufficient progress of the ferritetransformation in the two-phase zone during the subsequent coolingprocess, and thus a desired microstructure is not obtained. For thisreason, the finishing temperature has to be from the Ar₃ transformationtemperature to 100° C. above the Ar₃ transformation temperature.

[0098] If high-pressure descaling is applied to a slab after roughrolling, it is preferable that the value of the impact pressure P (MPa)of high pressure water on the steel sheet surface multiplied by the flowrate L (l/cm²) of the water is equal to or above 0.0025.

[0099] The impact pressure P of the high pressure water on a steel sheetsurface is expressed as follows (see the Tetsu-to-Hagane, 1991, vol. 77,No. 9, pl450):

P (MPa)=5.64×Po×V×H ²,

[0100] where Po (MPa) is the pressure of liquid, V (l/min.) is theliquid flow rate of a nozzle, and H (cm) is the distance between thenozzle and the steel sheet.

[0101] The flow rate L (l/cm²) is expressed as follows:

L (l/cm²)=V/(W×v),

[0102] where V (l/min.) is the liquid flow rate of a nozzle, W (cm) isthe width in which the liquid blown from a nozzle hits the steel sheetsurface and v (cm/min.) is the travelling speed of the steel sheet.

[0103] To obtain the effect of the present invention, no specific upperlimit has to be set regarding the value of the impact pressure Pmultiplied by the flow rate L, but it is preferable that the value is0.02 or below since, when the liquid flow rate of a nozzle is increased,troubles such as increased wear of the nozzle and the like will occur.

[0104] It is preferable, further, that the maximum surface roughness Ryof the steel sheet after the finish rolling is 15 μm (15 μmRy, 12.5 mm,ln12.5 mm) or less. The reason for this is clear from the fact that thefatigue strength of a steel sheet as hot rolled or pickled correlateswith the maximum roughness Ry of the steel sheet surface, as stated inpage 84 of Metal Material Fatigue Design Handbook edited by the Societyof Materials Science, Japan, for example. It is preferable that thefinish hot rolling is done within 5 sec. after the high pressuredescaling in order to prevent scale from forming again.

[0105] Immediately after the finish rolling, the steel sheet has to beheld in the temperature range from the Ar₃ transformation temperature tothe Ar₁ transformation temperature (the two-phase zone of ferrite andaustenite) for 1 to 20 sec. This retention is meant for acceleratingferrite transformation in the two-phase zone. If the retention time isless than 1 sec., the ferrite transformation in the two-phase zone isnot enough for obtaining a sufficient ductility and, if it exceeds 20sec., on the other hand, pearlite forms and the desired compoundstructure having ferrite as the main phase and martensite, or retainedaustenite mainly as the second phase, is not obtained.

[0106] It is preferable that the temperature range during the retentionfor 1 to 20 sec. is from the Ar₁ transformation temperature to 800° C.for the purpose of promoting the ferrite transformation. To this end, itis preferable to cool the steel sheet to this temperature range asquickly as possible at a cooling rate of 20° C./sec. or higher aftercompleting the finish rolling. Additionally, in order to avoid a drasticdecease in productivity, it is preferable that the retention time iscurtailed to 1 to 10 sec.

[0107] Then the steel sheet is cooled from the above temperature rangeto a coiling temperature (CT) at a cooling rate of 20° C./sec. orhigher. If the cooling rate is below 20° C./sec., pearlite or bainitecontaining much carbide form and martensite or retained austenite doesnot form in a sufficient amount and, consequently, the desiredmicrostructure having ferrite as the main phase and martensite orretained austenite as the second phase is not obtained.

[0108] The effect of the present invention can be enjoyed withoutbothering to specify an upper limit of the cooling rate during thecooling down to the coiling temperature but, to avoid the warping of asheet caused by thermal strain, it is preferable to control the coolingrate to 200° C./sec. or below.

[0109] The coiling temperature has to be 350° C. or below when producinga steel sheet whose microstructure is a compound structure havingferrite as the main phase and martensite as the second phase. The reasonfor this is that, if the coiling temperature is above 350° C., bainiteforms and martensite does not form in a sufficient amount, and thus thedesired microstructure having ferrite as the main phase and martensiteas the second phase is not obtained. Therefore, the coiling temperaturehas to be 350° C. or below. It is not necessary to specifically set alower limit of the coiling temperature but, to avoid a bad appearancecaused by rust when a coil is kept wet for a long period, it ispreferable that the coiling temperature is 50° C. or above.

[0110] When producing a steel sheet whose microstructure is a compoundstructure having ferrite as the main phase and the retained austenitewith a volume percentage of 5% or more and 25% or less as the secondphase, the coiling temperature has to be above 350° C. and 450° C. orbelow. The reason for this is that, if the coiling temperature exceeds450° C., bainite containing much carbide forms and retained austenitedoes not form in a sufficient amount, and thus the desiredmicrostructure is not obtained, and that, if the coiling temperature is350° C. or below, a large amount of martensite forms and retainedaustenite does not form in a sufficient amount, and thus the desiredmicrostructure is not obtained. The coiling temperature, therefore, hasto be above 350° C. and 450° C. or below.

[0111] In the present invention, a high fatigue strength steel sheet mayalso be a cold rolled steel sheet. In this case, although it is notnecessary to strictly specify the conditions of cold rolling afterpickling, it is preferable that the cold reduction rate is 30 to 80%.The reason for this is that, if the reduction rate is below 30%,recrystallization at the succeeding annealing process becomes incompleteand ductility is deteriorated, and that, if it is above 80%, the rollingload on a cold rolling mill becomes too high.

[0112] Finally, the present invention assumes that continuous annealingis employed in the annealing process. A steel sheet has to be heated tothe two-phase temperature range, namely from the Ac₁ temperature to theAc₃ temperature. However, it has to be noted that, if the heatingtemperature is too low even within the above temperature range and ifcementite has precipitated after hot rolling, it takes too long for thecementite to return to solid solution, and that, if the heatingtemperature is too high even within the above temperature range, thevolume percentage of austenite becomes too large, the carbonconcentration in the austenite decreases and the cooling curve in theCCT diagram tends to cross the transformation nose of bainite containingmuch carbide or that of pearlite. For this reason, it is preferable thatthe heating temperature is 780° C. or above and 850° C. or below. withregard to the retention time, a retention time below 15 sec. isinsufficient for the cementite to return to solid solution completelyand, if the retention time exceeds 600 sec., it requires an undesirablyslow travelling speed of the steel sheet. For the above reasons, theretention time has to be 15 to 600 sec. Then, for the cooling rate afterthe retention, when cooled at a rate below 20° C./sec., the coolingcurve in the CCT diagram tends to cross the transformation nose ofbainite containing much carbide or that of pearlite and, therefore, thecooling rate has to be 20° C./sec. or higher. If the cooling endtemperature is higher than 350° C., the desired microstructure is notobtained, and hence the steel sheet has to be cooled to a temperaturerange of 350° C. or lower.

[0113] Further, when producing a high fatigue strength cold rolled steelsheet having retained austenite as the second phase, the steel sheet hasto be held at a temperature of 350 to 450° C., namely a temperaturerange to accelerate bainite transformation and stabilize the retainedaustenite phase in a sufficient amount. If the holding temperature isabove 450° C., the retained austenite dissolves into pearlite. If it isbelow 350° C., fine carbide precipitates and the retained austenite doesnot form in a desired amount, causing deterioration of ductility. Forthe above reasons, the holding temperature to accelerate the bainitetransformation and stabilize the retained austenite in a sufficientamount is defined to be above 350° C. and 450° C. or lower. With regardto the retention time, if a retention time is below 15 sec., theacceleration of the bainite transformation is insufficient and unstableretained austenite transforms into martensite at the end of the cooling,and thus stable retained austenite phase is not obtained in a sufficientamount. If the retention time exceeds 600 sec., the bainitetransformation is accelerated too much and the stable retained austenitephase is not obtained in a sufficient amount. Another problem with thisis an undesirably slow travelling speed of the steel sheet. Theretention time to accelerate the bainite transformation and stabilizethe retained austenite phase in a sufficient amount is, therefore, 15sec. or longer and 600 sec. or shorter. Finally, as for the cooling rateto the cooling end temperature, if it is below 5° C./sec., the bainitetransformation is accelerated too much and the stable retained austenitephase may not be obtained in a sufficient amount. For this reason, thecooling rate has to be 5° C./sec. or more.

EXAMPLE 1

[0114] The present invention will be further explained based onexamples.

[0115] Steels A to Q having the respective chemical compositions listedin Table 1 were produced using a converter, and each of them underwentthe following production processes: continuous casting into slabs;reheating to the respective heating temperature (SRT) listed in Table 2,rough rolling and then finish rolling into a thickness of 1.2 to 5.4 mmat the respective final rolling temperature (FT) listed also in Table 2,and then coiling at the respective coiling temperature (CT) also listedin Table 2. Some of them underwent high pressure descaling under thecondition of an impact pressure of 2.7 MPa and a flow rate of 0.001l/cm² after the rough rolling.

[0116] The No. 5 test pieces according to JIS Z 2201 were cut out fromthe hot-rolled steel sheets thus produced and underwent a tensile testin accordance with the test method specified in JIS Z 2241. The testresult is shown in Table 2. Here, the volume percentages of ferrite andthe second phase are defined as their respective area percentages in themicrostructure observed with a light-optic microscope at a magnificationof 200 to 500 times at ¼ of the steel sheet thickness in a sectionsurface along the rolling direction. Note that the average grain size ofthe ferrite was measured in accordance with the section methodstipulated in the test method of ferrite crystal grain size of steelunder JIS G 0552, and that the average grain size of the second phasewas defined as the equivalent diameter of an average circle and thevalue obtained from an image processor and the like was used. Hardnesswas measured in accordance with the Vickers hardness test methodspecified in JIS Z 2244 under a testing force of 0.049 to 0.098 N and aretention time of 15 sec.

[0117] The carbon concentration in the second phase is the valueobtained by the calibration curve method described in the literature(Hiroyoshi Soejima: Electron Beam Micro Analysis, published from NikkanKogyo Shimbunsha) using an EPMA (electron probe micro analyzer). Notethat, because five or more of the second phase grains were measured, thecarbon concentration value is an average value of the measured grains.

[0118] Regarding some of the specimens A to Q, the carbon concentrationin the second phase was measured by the simplified measuring method.

[0119] Further, a fatigue test under completely reversed plane bendingwas conducted on the test pieces for plane bending fatigue test shown inFIG. 4 having a length of 98 mm, a width of 38 mm, a width of theminimum section portion of 20 mm and a notch radius of 30 mm. Thefatigue property of the steel sheets was evaluated in terms of thequotient of the fatigue limit σW after 10×10⁷ times of bending dividedby the tensile strength σB of the steel sheet (the above quotient beinga relative fatigue limit, expressed as σW/σB).

[0120] Note that no machining was done to the surfaces of the testpieces for the fatigue test and they were tested their surfaces left aspickled.

[0121] The burring workability (hole expansibility) was evaluatedfollowing the hole expanding test method according to the Standard ofthe Japan Iron and Steel Federation JFS T 1001-1996.

[0122] 11 steels, namely steels A, B, C-6, G, K, L, M, N, O, P and Q,conform to the present invention. In each of them, what was obtained wasthe compound structure steel sheet excellent in burring workabilityhaving: prescribed amounts of component elements; a microstructure of acompound structure having ferrite as the phase accounting for thelargest volume percentage and martensite mainly as the second phase; anaverage grain size of the ferrite being 2 μm or more and 20 μm or less;a quotient of the average grain size of the second phase divided by theaverage grain size of the ferrite being 0.05 or more and 0.8 or less; acarbon concentration in the second phase being 0.2% or more and 2% orless; a quotient of the volume percentage of the second phase Vs dividedby the average grain size of the second phase dm being 3 or more and 12or less; and a quotient of the average hardness of the second phase Hvsdivided by the average hardness of the ferrite Hvf being 1.5 or more and7 or less.

[0123] All the other steels fell outside the scope of the presentinvention for the following reasons:

[0124] In steel C-1, the final finish rolling temperature (FT) was abovethe range of the present invention and the grain size of the ferrite(Df), the size of the second phase (dm/Df), the carbon concentration inthe second phase (Cm) and the grain size of the second phase (Vs/dm)were outside the respective ranges of the present invention, and, as aresult, a sufficiently good value was not obtained in either the holeexpansion rate (γ) or the relative fatigue limit (σW/σB).

[0125] In steel C-2, the final finish rolling temperature (FT) was belowthe range of the present invention, and the size of the second phase(dm/Df) and the difference in strength between the ferrite and thesecond phase (Hvs/Hvf) were outside the respective ranges of the presentinvention and, consequently, a sufficiently good value was not obtainedin either the hole expansion rate (γ) or the relative fatigue limit(σW/σB). Besides, elongation (El) was low owing to residual strain.

[0126] In steel C-3, the cooling rate (CR) after the retention time wasslower than the range of the present invention and the coilingtemperature (CT) was higher than the range of the present invention and,as a consequence, the grain size of the ferrite (Df), the size of thesecond phase (dm/Df), the carbon concentration in the second phase (Cm)and the grain size of the second phase (Vs/dm) were outside therespective ranges of the present invention. As a result, a sufficientlygood value was not obtained in either the hole expansion rate (λ) or therelative fatigue limit (σW/σB).

[0127] In steel C-4, the retention temperature (MT) after the finishrolling and before the coiling was below the range of the presentinvention, and the size of the second phase (dm/Df), the carbonconcentration in the second phase (Cm) and the strength differencebetween the ferrite and the second phase (Hvs/Hvf) were outside therespective ranges of the present invention and, as a result, asufficiently good value was not obtained in either the hole expansionrate (λ) or the relative fatigue limit (σW/σB).

[0128] In steel C-5, no retention time (Time) was secured between thefinish rolling and the coiling, and the size of the second phase(dm/Df), the carbon concentration in the second phase (Cm) and thestrength difference between the ferrite and the second phase (Hvs/Hvf)were outside the respective ranges of the present invention and,consequently, a sufficiently good value was not obtained in either thehole expansion rate (λ) or the relative fatigue limit (σW/σB).

[0129] In steel D, the desired microstructure was not obtained becausethe C content was outside the range of the present invention and, as aresult, a sufficiently good value was not obtained in either thestrength (TS) or the relative fatigue limit (σW/σB).

[0130] In steel E, the content of Si was outside the range of thepresent invention and, consequently, a sufficiently good value was notobtained in either the strength (TS) or the relative fatigue limit(σW/σB).

[0131] In steel F, the content of Mn was outside the range of thepresent invention, and the grain size of the ferrite (Df), the size ofthe second phase (dm/Df) and the grain size of the second phase (Vs/dm)were outside the respective ranges of the present invention and, as aresult, a sufficiently good value was not obtained in any of thestrength (TS), the hole expansion rate (λ) and the relative fatiguelimit (σW/σB).

[0132] In steel H, the content of S was outside the range of the presentinvention and, as a result, a sufficiently good value was not obtainedin either the hole expansion rate (λ) or the relative fatigue limit(σW/σB).

[0133] In steel I, the content of P was outside the range of the presentinvention and, consequently, a sufficiently good value was not obtainedin the relative fatigue limit (σW/σB).

[0134] In steel J, the content of C was outside the range of the presentinvention and, as a result, a sufficiently good value was not obtainedin any of the elongation(El), the hole expansion rate (λ) and therelative fatigue limit (σW/σB). TABLE 1 Chemical composition (in mass %)Steel C Si Mn P S Al Others Remark A 0.055 0.890 1.21 0.008 0.0006 0.032Inventive example B 0.047 1.640 1.21 0.007 0.0008 0.025 Inventiveexample C 0.074 1.620 1.79 0.009 0.0009 0.026 Inventive example D 0.0030.120 0.24 0.080 0.0008 0.019 Comparative example E 0.045 0.006 1.220.011 0.0011 0.030 Comparative example F 0.055 0.780 0.03 0.012 0.00080.033 Comparative example G 0.067 1.590 1.48 0.009 0.0007 0.032 Cu:1.18, Ni: 0.62, B: 0.0002 Inventive example H 0.070 1.660 1.81 0.0080.0300 0.028 Comparative example I 0.071 1.610 1.81 0.180 0.0010 0.025Comparative example J 0.250 0.880 1.11 0.080 0.0008 0.027 Comparativeexample K 0.072 1.610 1.82 0.009 0.0011 0.030 Ca: 0.0008 Inventiveexample L 0.120 0.910 1.51 0.008 0.0013 0.038 Ti: 0.08 Inventive exampleM 0.081 1.881 1.60 0.007 0.0010 0.036 Nb: 0.03 Inventive example N 0.0681.630 0.21 0.008 0.0009 0.022 Mo: 0.63 Inventive example O 0.066 1.2102.11 0.077 0.0009 0.023 V: 0.07 Inventive example P 0.051 0.263 1.330.009 0.0011 0.026 Cr: 0.11 Inventive example Q 0.038 0.880 1.31 0.0100.0012 0.028 Zr: 0.05, REM: 0.0006 Inventive example

[0135] TABLE 2 Microstructure Mar- Production condition Ferr- ten- Bai-Second SRT FT MT Time CR CT ite site nite Cm Df phase* Vs/ Hvs/ Steel (°C.) (° C.) (° C.) (s) (° C./s) (° C.) (%) (%) (%) (%) (μm) dm/Df (%) dmHvf A 1200 860 680 5 90 50 93 7 0 0.76 15 0.08 7 (7) 5.8 6.3 B 1150 870650 5 90 50 88 12 0 0.36 12 0.15 12 (12) 6.7 3.3 C-1 1150 910 670 5 9050 60 10 30 0.15 21 0.90 40 (10) 2.1 1.9 C-2 1150 740 600 5 90 50 70 1020 0.22 10 0.90 30 (10) 3.3 1.4 C-3 1150 820 600 5  5 550 40 0 60 0.1226 1.50 60 (0)  1.5 1.7 C-4 1150 830 400 5 90 50 45 0 55 0.09 7 1.20 55(0)  6.5 1.2 C-5 1150 810 — 0 90 50 50 0 50 0.12 6 1.00 50 (0)  8.3 1.2C-6 1150 820 620 5 90 50 85 15 0 0.46 9 0.25 15 (15) 6.7 3.4 D 1200 900720 5 90 50 100 0 0 — 60 — 0 (0) — — E 1200 860 650 5 90 50 90 3 7 0.4218 0.10 10 (3)  5.6 5.3 F 1200 860 640 5 90 50 83 0 17 0.20 28 0.04 17(0)  16.3  5.5 G 1150 810 610 5 90 50 85 12 3 0.42 6 0.30 15 (12) 8.33.4 H 1150 810 620 8 60 50 85 13 2 0.44 8 0.20 15 (13) 9.4 3.2 I 1150810 630 8 60 50 84 16 0 0.41 7 0.20 16 (16) 11.4  3.1 J 1200 800 700 860 50 85 25 20 0.68 — — 45 (25) — — K 1150 810 610 8 60 50 85 13 2 0.458 0.20 15 (13) 9.4 3.3 L 1250 810 680 8 60 50 75 20 5 0.45 11 0.35 25(10) 8.5 4.0 M 1150 810 680 8 60 50 82 16 2 0.42 9 0.25 18 (16) 8.0 3.1N 1150 810 610 8 60 50 90 10 0 0.65 16 0.20 10 (10) 3.1 6.5 O 1150 810680 8 60 50 82 15 3 0.34 10 0.25 18 (15) 7.2 2.8 P 1200 820 670 8 60 5094 6 0 0.82 17 0.07 6 (8) 5.0 6.1 Q 1200 840 670 8 60 50 94 6 0 0.60 150.07 6 (6) 5.7 5.2 Fatigue property Mechanical properties σW/ σY σB YREl λ σW σB Steel (MPa) (MPa) (%) (%) (%) (MPa) (%) Remark A 388 607 6434 86 320 53 Inventive example B 426 699 61 32 79 365 52 Inventiveexample C-1 653 845 77 19 29 380 45 Comparative example C-2 675 820 8215 34 360 44 Comparative example C-3 562 733 77 28 33 330 45 Comparativeexample C-4 688 875 79 19 30 400 46 Comparative example C-5 551 810 6820 39 350 43 Comparative example C-6 485 783 62 28 75 410 52 Inventiveexample D 194 324 60 45 116 150 46 Comparative example E 367 496 74 3556 200 40 Comparative example F 323 521 62 35 34 245 47 Comparativeexample G 505 789 64 27 62 450 57 Inventive example H 498 790 63 21 19370 47 Comparative example I 518 836 62 22 49 355 42 Comparative exampleJ 742 1160 64 11 5 450 39 Comparative example K 479 786 61 27 61 410 52Inventive example L 469 722 65 26 70 370 51 Inventive example M 528 81265 23 64 420 52 Inventive example N 345 556 62 34 90 280 50 Inventiveexample O 525 821 64 22 65 430 52 Inventive example P 337 561 60 35 92290 52 Inventive example Q 387 624 62 32 83 320 51 Inventive example

EXAMPLE 2

[0136] The present invention will further be explained hereafter basedon other examples.

[0137] Steels A to O having the respective chemical compositions listedin Table 3 were produced using a converter, and each of them underwentthe following production processes: continuous casting into slabs;reheating to the respective heating temperature (SRT) listed in Table 4,rough rolling and then finish rolling into a thickness of 1.2 to 5.4 mmat the respective final rolling temperature (FT) listed also in Table 4,and then coiling at the respective coiling temperature (CT) also listedin Table 4. Some of them underwent a high pressure descaling under thecondition of an impact pressure of 2.7 MPa and a flow rate of 0.001l/cm² after the rough rolling. TABLE 3 Chemical composition (in mass %)No Steel C Si Mn P S Al Others Remark  1 A 0.100 1.360 1.32 0.008 0.00060.032 Inventive example  2 B 0.003 0.120 0.24 0.080 0.0008 0.019Comparative example  3 C 0.090 0.007 1.35 0.010 0.0007 0.030 Comparativeexample  4 D 0.120 1.400 0.02 0.007 0.0008 0.031 Comparative example  5E 0.150 1.920 1.46 0.010 0.0010 0.036 CU: 0.58, Ni: 0.23, B: 0.0002Inventive example  6 F 0.168 1.950 1.60 0.150 0.0010 0.041 Comparativeexample  7 G 0.170 1.900 1.55 0.008 0.0300 0.035 Comparative example  8H 0.310 1.350 1.30 0.012 0.0011 0.041 Comparative example  9 I 0.1161.880 1.66 0.011 0.0006 0.032 Ca: 0.0009 Inventive example 10 J 0.1551.910 1.60 0.010 0.0007 0.030 Ti: 0.07 Inventive example 11 K 0.1711.790 1.75 0.008 0.0008 0.040 Nb: 0.03 Inventive example 12 L 0.1681.900 1.55 0.007 0.0007 0.041 Mn: 0.61 Inventive example 13 M 0.0951.400 1.35 0.013 0.0007 0.044 V: 0.07 Inventive example 14 N 0.110 1.3501.40 0.007 0.0009 0.021 Cr: 0.12 Inventive example 15 O 0.100 1.330 1.440.011 0.0012 0.026 Zr: 0.05, REM: 0.0004 Inventive example

[0138] The No. 5 test pieces according to JIS Z 2201 were cut out fromthe hot-rolled steel sheets thus produced and underwent a tensile testin accordance with the test method specified in JIS Z 2241. The testresult is shown in Table 4. “Others” in “Micro structure” of Table 4indicates pearlite or martensite. Here, the volume percentages of theretained austenite, ferrite, bainite, pearlite and martensite aredefined as the respective area percentages observed with a light-opticmicroscope at a magnification of 200 to 500 times in the microstructureon the section surface at ¼ of the sheet thickness of the specimens cutout from the ¼ or ¾ width position of the steel sheets, after polishingthe section surface along the rolling direction and etching it with anitral reagent and a reagent disclosed in Japanese Unexamined PatentPublication No. H5-163590. However, some of the figures are thoseobtained by the X-ray diffraction method. The average grain size of theretained austenite was defined as the equivalent diameter of an averagecircle and the value obtained from an image processor and the like wasused. Hardness was measured in accordance with the Vickers hardness testmethod specified in JIS Z 2244 under a testing force of 0.049 to 0.098 Nand a retention time of 15 sec.

[0139] Further, a fatigue test under completely reversed plane bendingwas conducted on the test pieces for plane bending fatigue test shown inFIG. 4 having a length of 98 mm, a width of 38 mm, a width of theminimum section portion of 20 mm and a notch radius of 30 mm. Thefatigue property of the steel sheets was evaluated in terms of thequotient of the fatigue limit σ_(W) after 10×10⁷ times of bendingdivided by the tensile strength σ_(B) of the steel sheet (the abovequotient being a relative fatigue limit, expressed as σ_(W)/σ_(B)). Notethat no machining was done to the surfaces of the test pieces for thefatigue test and they were tested with their surfaces left as pickled.

[0140] The burring workability (hole expansibility) was evaluated interms of the hole expansion value obtained by the hole expanding testmethod according to the Standard of the Japan Iron and Steel FederationJFS T 1001-1996. TABLE 4 Microstructure Reta- ined Production conditionFerr- Bai- auste- SRT FT MT Time CR CT ite nite nite Others Vs/ Hvs/ NoSteel (° C.) (° C.) (° C.) (s) (° C./s) (° C.) (%) (%) (%) (%) dm Hvf  1A-1 1200 850 660 8 90 380 85 5 10 0 3.3 3.1  2 A-2 1200 740 660 8 90 38085 10 5 0 2.4 2.8  3 A-3 1200 920 660 8 90 380 65 35 0 0 — —  4 A-4 1200850 540 8 90 380 35 65 0 0 — —  5 A-5 1200 850 720 8 90 380 60 30 0 10 ——  6 A-6 1200 850 — 0 90 380 60 40 0 0 — —  7 A-7 1200 850 660 8  5 38080 10 0 10 —  8 A-8 1200 850 660 8 90 550 80 20 0 0 — —  9 A-9 1200 850660 8 90 150 85 5 3 7 1.5 2.0 10 B 1200 900 720 5 90 400 100 0 0 0 — 11C 1150 810 620 8 90 400 40 60 0 0 — — 12 D 1150 830 650 8 90 400 80 17 30 1.5 7.6 13 E 1150 820 630 8 90 410 70 15 15 0 3.8 2.2 14 F 1150 820630 8 90 410 72 18 10 0 5.1 2.6 15 G 1150 820 630 8 90 410 66 18 16 04.9 2.3 16 H 1150 800 620 8 90 410 35 45 20 0 6.5 3.2 17 I 1150 820 6308 90 410 68 16 16 0 8.6 2.0 18 J 1150 820 630 8 90 410 71 14 15 0 7.92.1 19 K 1150 820 630 8 90 410 70 15 15 0 7.8 2.3 20 L 1150 820 630 8 90410 72 15 13 0 7.2 2.3 21 M 1200 850 650 5 55 390 85 7 8 0 3.1 1.8 22 N1200 850 650 5 55 390 83 6 11 0 3.8 1.9 23 O 1200 850 650 5 55 390 83 710 0 3.7 1.8 Fatigue Mechanical properties property σY σB El TS × El λσW σW/σB No (MPa) (MPa) (%) (MPa · %) (%) (MPa) (%) Remark 1 439 617 3722829 82 325 53 Inventive example 2 555 631 25 15775 42 320 51Comparative example 3 491 622 25 15550 70 300 48 Comparative example 4620 703 21 14763 85 300 43 Comparative example 5 480 620 21 13020 36 28045 Comparative example 6 505 644 23 14812 76 300 74 Comparative example7 472 588 24 14112 48 280 48 Comparative example 8 477 596 26 15496 90290 49 Comparative example 9 435 650 30 19500 78 330 51 Comparativeexample 10 194 334 43 14362 121 150 45 Comparative example 11 408 526 2915254 42 245 47 Comparative example 12 421 544 27 14688 38 250 46Comparative example 13 583 789 30 23670 61 440 56 Inventive example 14592 822 28 23016 28 380 46 Comparative example 15 603 815 23 18745 22370 45 Comparative example 16 854 1073 11 11803 16 450 42 Comparativeexample 17 548 769 31 23839 70 385 50 Inventive example 18 590 786 3023580 66 390 50 Inventive example 19 620 826 28 23128 62 425 51Inventive example 20 584 811 28 22708 60 420 52 Inventive example 21 449607 36 21852 78 320 53 Inventive example 22 450 641 35 22435 75 340 53Inventive example 23 447 621 34 21114 86 330 53 Inventive example

[0141] 9 steels, namely steels A-1, E, I, J, K, L, M, N and O conform tothe present invention. In each of them, what was obtained was awork-induced transformation type compound structure steel sheetexcellent in burring workability characterized by having: prescribedamounts of component elements; a microstructure of a compound structurecontaining retained austenite accounting for a volume percentage of 5%or more and 25% or less and the balance consisting mainly of ferrite andbainite; a quotient of the volume percentage of the retained austenitedivided by its average grain size being 3 or more and 12 or less; and aquotient of the average hardness of the retained austenite divided bythe average hardness of the ferrite being 1.5 or more and 7 or less.

[0142] All the other steels fell outside the scope of the presentinvention for the following reasons.

[0143] In steel A-2, the final finish rolling temperature (FT) was belowthe range of the present invention and, as a result, both astrength-ductility balance (TS×El) and the hole expansion rate (λ) werelow owing to residual strain. In steel A-3, the final finish rollingtemperature (FT) was above the range of the present invention and thusthe desired microstructure was not obtained and, as a result, both thestrength-ductility balance (TS×El) and the relative fatigue limit(σ_(W)/σ_(B)) were low. In steel A-4, the retention temperature (MT)after finish rolling and before coiling was below the range of thepresent invention and thus the desired microstructure was not obtainedand, consequently, both the strength-ductility balance (TS×El) and therelative fatigue limit (σ_(S)/σ_(B)) were low.

[0144] In steel A-5, the retention temperature (MT) after finish rollingand before coiling was above the range of the present invention and thusthe desired microstructure was not obtained, and consequently, both thestrength-ductility balance (TS×El) and the relative fatigue limit(σ_(W)/σ_(B)) were low. In steel A-6, no retention time (Time) wassecured between finish rolling and coiling and thus the desiredmicrostructure was not obtained and, as a result, both thestrength-ductility balance (TS×El) and the relative fatigue limit(σ_(W)/σ_(B)) were low. A sufficient value of hole expansion rate (λ)was not obtained, either. In steel A-7 the cooling rate (CR) after theretention was slower than the range of the present invention and thusthe desired microstructure was not obtained and, as a result, both thestrength-ductility balance (TS×El) and the relative fatigue limit(σ_(W)/σ_(B)) were low. A sufficient value of hole expansion rate (λ)was not obtained, either. In steel A-8, the coiling temperature (CT) wasabove the range of the present invention and thus the desiredmicrostructure was not obtained and, consequently, thestrength-ductility balance (TS×El) was low. In steel A-9, the coilingtemperature (CT) was below the range of the present invention and thusthe desired microstructure was not obtained and, as a result, thestrength-ductility balance (TS×El) was low.

[0145] In steel B, the desired microstructure was not obtained becausethe C content was outside the range of the present invention and, as aresult, a sufficiently good value was not obtained in either thestrength (TS) or the relative fatigue limit (σ_(W)/σ_(B)). In steel C,the content of Si was outside the range of the present invention and, asa result, a sufficiently good value was not obtained in either thestrength (TS) or the relative fatigue limit (σ_(W)/σ_(B))- In steel D,the content of Mn was outside the range of the present invention andthus the desired microstructure was not obtained and, as a result, boththe strength-ductility balance (TS×El) and the relative fatigue limit(σ_(W)/σ_(B)) were low. In steel F, the content of P was outside therange of the present invention and, as a result, a sufficiently goodvalue was not obtained in the relative fatigue limit (σ_(W)/σ_(B)). Insteel G, the content of S was outside the range of the present inventionand, as a result, a sufficiently good value was not obtained in eitherthe hole expansion rate (λ) or the relative fatigue limit (σ_(W)/σ_(B)).In steel H, the C content was outside the range of the present inventionand, as a result, a sufficiently good value was not obtained in any ofthe elongation (El), the hole expansion rate (λ) and the relativefatigue limit

Industrial Applicability

[0146] As heretofore described in detail, the present invention providesa compound structure steel sheet excellent in burring workability havinga tensile strength of 540 MPa or more, and a method to produce the same.The hot-rolled steel sheet according to the present invention realizes aremarkable improvement in burring workability (hole expansibility) whilemaintaining a sufficiently good fatigue property and, therefore, thepresent invention has a high industrial value.

1. A high fatigue strength steel sheet excellent in burring workabilitycharacterized in that: the steel sheet is made of a steel containing, bymass, 0.01 to 0.3% of C, 0.01 to 2% of Si, 0.05 to 3% of Mn, 0.1% orless of P, 0.01% or less of S, and 0.005 to 1% or Al, and the balanceconsisting of Fe and unavoidable impurities; the microstructure is acompound structure having ferrite as the main phase and martensite asthe second phase; the average grain size of the ferrite is 2 μm or moreand 20 μm or less; the quotient of the average grain size of the secondphase divided by the average grain size of the ferrite is 0.05 or moreand 0.8 or less; and the carbon concentration in the second phase is0.2% or more and 3% or less.
 2. A high fatigue strength steel sheetexcellent in burring workability characterized in that: the steel sheetis made of a steel containing, by mass, 0.01 to 0.3% of C, 0.01 to 2% ofSi, 0.05 to 3% of Mn, 0.1% or less of P, 0.01% or less of S, and 0.005to 1% or Al, and the balance consisting of Fe and unavoidableimpurities; the microstructure is a compound structure having ferrite asthe main phase and martensite as the second phase; the quotient of thevolume percentage of the second phase divided by its average grain sizeis 3 or more and 12 or less; and the quotient of the average hardness ofthe second phase divided by the average hardness of the ferrite is 1.5or more and 7 or less.
 3. A high fatigue strength steel sheet excellentin burring workability according to claim 1 or 2, characterized in that;the steel further contains, in mass, 0.2 to 2% of Cu, and the Cu existsin the ferrite phase of the steel in the state of the precipitates ofgrains 2 nm or less in size consisting purely of Cu and/or in the stateof solid solution.
 4. A high fatigue strength steel sheet excellent inburring workability according to any one of claims 1 to 3, characterizedby further containing, by mass, 0.0002 to 0.002% of B.
 5. A high fatiguestrength steel sheet excellent in burring workability according to anyone of claims 1 to 4, characterized by further containing, by mass, 0.1to 1% of Ni.
 6. A high fatigue strength steel sheet excellent in burringworkability according to any one of claims 1 to 5, characterized byfurther containing, by mass, one or both of 0.0005 to 0.002% of Ca and0.0005 to 0.02% of REM.
 7. A high fatigue strength steel sheet excellentin burring workability according to any one of claims 1 to 6,characterized by further containing, by mass, one or more of; 0.05 to0.5% of Ti, 0.01 to 0.5% of Nb, 0.05 to 1% of Mo, 0.02 to 0.2% of V,0.01 to 1% of Cr, and 0.02 to 0.2% of Zr.
 8. A high fatigue strengthsteel sheet excellent in burring workability according to any one ofclaims 1 to 7, characterized in that the microstructure is a compoundstructure having ferrite as the main phase and retained austeniteaccounting for a volume percentage of 5% or more and 25% or less as thesecond phase.
 9. A method to produce a high fatigue strength steel sheetexcellent in burring workability according to any one of claims 1 to 7,characterized by, when hot rolling a slab having said chemicalcomposition, completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of 350° C. or lower.
 10. A method to produce a high fatiguestrength steel sheet excellent in burring workability according to anyone of claims 1 to 7, characterized by, when hot rolling a slab havingsaid chemical composition, applying high pressure descaling to the slabafter rough rolling, completing finish hot rolling at a temperature fromthe Ar₃ transformation temperature to 100° C. above the Ar₃transformation temperature, holding the hot-rolled steel sheet thusproduced in the temperature range from the Ar₁ transformationtemperature to the Ar₃ transformation temperature for 1 to 20 sec., thencooling it at a cooling rate of 20° C./sec. or higher, and coiling it ata coiling temperature of 350° C. or lower.
 11. A method to produce ahigh fatigue strength steel sheet excellent in burring workabilityaccording to any one of claims 1 to 7, characterized by completing thehot rolling of a slab having said chemical composition at a temperatureof the Ar₃ transformation temperature or higher, subsequently picklingand cold-rolling the hot-rolled steel sheet thus produced, holding thecold-rolled steel sheet in the temperature range from the Ac₁transformation temperature to the AC₃ transformation temperature for 30to 150 sec., then cooling it at a cooling rate of 20° C./sec. or higherto the temperature range of 350° C. or lower.
 12. A method to produce ahigh fatigue strength steel sheet excellent in burring workabilityaccording to any one of claims 1 to 7, characterized by, when hotrolling a slab having said chemical composition, completing finish hotrolling at a temperature from the Ar₃ transformation temperature to 100°C. above the Ar₃ transformation temperature, holding the hot-rolledsteel sheet thus produced in the temperature range from the Ar₁transformation temperature to the Ar₃ transformation temperature for 1to 20 sec., then cooling it at a cooling rate of 20° C./sec. or higher,and coiling it at a coiling temperature of above 350° C. and 450° C. orlower.
 13. A method to produce a high fatigue strength steel sheetexcellent in burring workability according to any one of claims 1 to 7,characterized by, when hot rolling a slab having said chemicalcomposition, applying high pressure descaling to the slab after roughrolling, completing finish hot rolling at a temperature from the Ar₃transformation temperature to 100° C. above the Ar₃ transformationtemperature, holding the hot-rolled steel sheet thus produced in thetemperature range from the Ar₁ transformation temperature to the Ar₃transformation temperature for 1 to 20 sec., then cooling it at acooling rate of 20° C./sec. or higher, and coiling it at a coilingtemperature of above 350° C. and 450° C. or lower.
 14. A method toproduce a high fatigue strength steel sheet excellent in burringworkability according to any one of claims 1 to 7, characterized by,completing the hot rolling of a slab having said chemical composition ata temperature of the Ar₃ transformation temperature or higher,subsequently pickling and cold rolling the hot-rolled steel sheet thusproduced, holding the cold-rolled steel sheet in the temperature rangefrom the Ac₁ transformation temperature to the AC₃ transformationtemperature for 30 to 150 sec., then cooling it at a cooling rate of 20°C./sec. or higher, holding it in the temperature range of above 350° C.and 450° C. or lower for 15 to 600 sec., and cooling it at a coolingrate of 5° C./sec. or higher to the temperature range of 150° C. orbelow.